1. Field
The present application relates generally to methods and apparatus for initialization of a wellbore survey tool.
2. Description of the Related Art
There are typically two types of surveying by which wellbore survey tools conduct surveys (e.g., gyroscopic- or gyro-based surveys) of wellbores. The first type is static surveying, in which measurements of the Earth's rotation are taken at discrete depth intervals along the well trajectory. These measurements can be used to determine the orientation of the survey tool with respect to a reference vector, such as the vector defined by the horizontal component of the Earth's rate in the direction of the axis of the Earth's rotation; a process also referred to herein as gyro-compassing. The second type is continuous surveying, in which the gyroscopic or gyro measurements are used to determine the change in orientation of the survey tool as it traverses the well trajectory. This process uses the gyro measurements of turn rate with respect to a known start position. The start position may be derived, for example, by conducting a static survey prior to entering the continuous survey mode (which may also be referred to as an autonomous or autonomous/continuous survey mode).
Under certain circumstances, static surveying generally becomes less accurate than in other circumstances. For example, when operating at high latitudes on the Earth's surface the static survey process becomes less accurate than at low latitudes. At relatively high latitudes, the reference vector to which the survey tool aligns itself during the gyro-compassing procedure, the horizontal component of Earth's rate (ΩH), is small compared to the value in equatorial and mid-latitude regions, as indicated by the following equation:ΩH=Ω cos L,  (Eq. 1)where Ω=Earth's rate and L=latitude. Generally, a satisfactory directional survey can be achieved using gyro-compassing at latitudes of up to about 60 degrees. However, the accuracy can degrade rapidly thereafter as the cosine of latitude reduces more rapidly and the magnitude of ΩH thus becomes much smaller. FIG. 1 schematically illustrates the horizontal component ΩH of the Earth's rate for changing latitude. As shown, at zero latitude ΩH is at its maximum value and is equal to the Earth's rate (Ω). ΩH successively decreases to ΩH=Ω cos L1 and ΩH=Ω cos L2 for increasing latitudes L1 and L2, respectively, and ΩH is zero at 90 degrees of latitude (i.e., at the North Pole). There is a significant amount of oil and gas exploration at relatively high latitudes (e.g., latitudes in excess of 70 degrees). At these latitudes, the accuracy of well surveys based on gyro-compassing can be degraded. Similar degradations in survey accuracy can also occur when using magnetic survey tools instead of, or in addition to, gyro-based survey tools. As such, survey accuracy may similarly decrease at locations close to the Earth's magnetic poles when using magnetic survey tools.
In addition, the accuracy of gyro-compassing can be degraded when conducted from a moving platform (e.g., an offshore platform), as compared to being conducted from a relatively static platform. For example, during operation from a moving platform, the survey tool will be subjected to platform rotational motion in addition to the Earth's rotation. Under such conditions, tool orientation with respect to the horizontal Earth's rate vector (ΩH) may be difficult to determine with the precision that is possible on a stationary platform since the directional reference, defined by ΩH is effectively corrupted by the platform motion.